In a hydraulic circuit in a conventional oil hydraulic press, after an operation for rotating a servo motor to feed oil into a two-way pump is performed, the pressure of a cylinder rod-side cylinder chamber is in a state in which the pressure becomes constant by a counterbalance valve. On the other hand, the pressure of a two-way pump-side piping which connects a speed switching valve to the two-way pump is almost in an atmospheric pressure state.
Due to this, if the speed switching valve is switched, the pressure difference between before and after the switching of the valve causes an upper table to fall downward by several millimeters with quite a great impact. This phenomenon is not limited to the above-stated operation. If the upper table is stopped at an upper limit position and left as it is for a while, the pressure of the two-way pump-side piping is decreased by leakage since the two-way pump is also stopped while the upper table is stopped. If the speed switching valve is to be turned on in that state, the upper table disadvantageously, suddenly lowers.
The present invention has been made while paying attention to these conventional disadvantages, and the first object of the invention is to provide an oil hydraulic press stop control method and a controller therefor capable of eliminating an impact which is generated when a speed switching valve is switched while a ram is stopped.
In addition, the second object of the present invention has been made while paying attention to these disadvantages of the conventional art and is to provide a speed switching valve fault detection method and a detector therefor for a hydraulic press capable of detecting the fault of a speed switching valve and ensuring operation safety.